Screenshot of Mugen*
As you have no doubt sassed, or at least heard of, there is a genre known as the "Fighting Game" - essentially a side-scrolling game where you fight alone or in teams agaisnt other teams using martial arts moves and specials such as fireballs and gravity-defying kicks.

What if there was a way to have all the fighters you want- but none that you dont- in a single game? Even from other fighting games, original characters, and even be able to create your own?

There is..

It's called M.U.G.E.N.

MUGEN is a fighting game engine for the PC- and it's extremly popular. The major haul is that it's extremly customizable* - one can go to a site such as Mugen4Ever and download 100s of characters. Dragon Ball Z characters, SNK Characters, capcom characters, even original characters and the ship from Darius Twin are available.

MUGEN also supports some very keen 2on2* modes- which can get crazy when the supers* start to fly.

MUGEN (by Elecbyte) comes with only minimal playability* but with all the characters already out there (most from other fighting games*) it's really no problem.

And don't worry- there are enough tutorials, FAQs, and other such things around that you'll be performing arial raves* in no time!